(1) Field of the Invention
The present invention relates to a fluid separation apparatus constructed by using permselective hollow fibers.
(2) Description of the Prior Art
Methods for separating substances by utilizing the permselective characteristics of a semipermeable membrane, such as reverse osmosis, ultrafiltration, osmosis, dialysis and pressure separation of gas mixtures, are known. A fluid separation apparatus comprising semipermeable membranes in the form of hollow fibers having an outer diameter of several hundred microns or less, is very advantageous as the fluid separation apparatus utilizing a permoselective membrane, because the weight of the semipermeable membrane or equivalent pressure externally applied is supported by hollow fibers per se having a tubular structure and owing to a very fine size of the hollow fibers, it is able to enlarge the effective surface area of the semipermeable membrane to be packed in the unit volume.
In order to prepare such a separation apparatus having a large permeation area by utilizing the abovementioned advantage, it is necessary to make a bundle of a very great number of hollow fibers and to fix each hollow fiber at the both ends, respectively, in a cylindrical casing by the tube sheets into which the every ends of the hollow fibers are potted without any opening and every bore of hollow fibers are are opened to the outsides of tube sheets of both bundle ends, so that the fluid circulating in the interior of the hollow fibers of thus assembled apparatus should never be mingled with the externally circulating fluid and every hollow fiber membrane will be effectively utilized. Processes for the preparation of such apparatus are disclosed in the specifications of U.S. Pat. Nos. 3,228,876 and 3,228,877.
Furthermore, the specification of U.S. Pat. No. 3,339,341 proposes a process comprising assembling a fibrous bundle composed of a great number of hollow fibers into a centrifugal apparatus and forming tube sheets on both the ends of the hollow fiber bundle potted by a curable liquid material under application of a centrifugal force. However, it is required a very high technique to make tube sheets by potting both the ends of bundle by such material without clogging and leakage, which bundle ranges in fiber quantity from several thousand hollow fibers to scores of millions of hollow fibers and which fibers are replasticized by glycerin or the like to prevent degradation of the semipermeable characteristics. Accordingly, it is practically very difficult to attain a high manufacturing efficiency by such potting processes.
Moreover, in a separation apparatus comprising a bundle of a very great number of fine hollow fibers, distribution of the circulating fluid in the external portions of the fibers in the interior of the apparatus frequently becomes non-uniform to form stagnant zones, and the separation efficiency of the apparatus is reduced considerably, or the flashing effect is locally insufficient when the apparatus is washed. And it results in serious defects in practical operation. For example, in case of hemo-dialysis, harmful substances such as a sterilizing agents are left in the apparatus, or in case of reverse osmosis or ultrafiltration, the performances of the apparatus may reduce every time when reversely flashing to regenerate.
Many attempts have heretofore been made to prevent formation of stagnant zones causing reduction of the separation efficiency and the capacity of the apparatus. For example, there has been proposed an apparatus in which ribbon-like members are wound around bundles of hollow fibers to prevent formation of stagnant zones between the bundles. However, the separation capacity of an apparatus of this type is insufficient and the apparatus is hardly put into practical use. Furthermore, the specification of U.S. Pat. No. 4,066,553 proposes an apparatus in which thread-like elements are wound around relatively thick tubular membranes composed of a porous support so that a certain space is maintained between such tubular membranes and the adjacent portion (wall) of the apparatus. However, this proposal is very difficult to apply to fine hollow fibers, which are soft and weak such an arrangement, and the improvement of the performances are hardly expected since the thread-like elements wound around tubular membranes are ordinary straight yarns.
Under such background, we conducted research with a view to developing a fluid separation apparatus capable of performing the fluid separation at a high efficiency and of being assembled very easily at a low cost, and as a result, we have now completed the present invention.